1. Field of the Invention
The invention is related to the field of communications, and in particular, to methods and apparatuses for expanding and retracting a frame selection interval in a wireless communication network.
2. Statement of the Problem
Spread spectrum wireless communication networks, such as code division multiple access (CDMA) 3G1x and EVDO, utilize soft handoffs (SHO) for voice and packet data applications. A mobile communication device engaged in a voice or data call communicates with one or more base stations simultaneously in a state known as SHO. Each base station has a call leg over a backhaul that transports voice traffic or data traffic as a sequence of frames (known as reverse frames) from the base station to a mobile switching center (MSC). Each base station transmits essentially the same frame data corresponding to the same digitized portion of a call to the MSC. The MSC has a frame selector that selects the “best” frame received from the base stations associated with the call during each instance of a frame selection interval (e.g., every 20 ms). The frame selector then transmits the best frame to other higher protocol layers (e.g., other network elements) or peer layers of the wireless communication network, such as a vocoder (for a circuit voice call), a Radio Link Protocol (RLP) entity (for a packet data call), or another frame selector (for a Transcoder Free Operation or TrFO call), for further processing and/or transmission to the destination of the call.
The frame selector further transmits a sequence of frames (known as forward frames) corresponding to the other end of the call to each base station during a similar interval period (e.g., 20 ms) for transmission to the mobile communication device. Included in the forward frames is information regarding the “best” frame (i.e., the best leg of the SHO) selected of the reverse frames. The base stations use this feedback information to perform power control (i.e., adjusting their transmission power), which allows spread spectrum systems (e.g., CDMA) to achieve maximum air-interface efficiency and call capacity.
The frame selector expects reverse frames from all of the call legs to arrive within a tight selection interval during each instance of the frame selection intervals. This means that the differential delay between the first frame received during an instance of the selection interval and the last frame received during an instance of the selection interval cannot exceed a certain limit for the frame selector to operate properly. Otherwise, the frame selector cannot compare a frame received from a first base station to a frame received from a second base station to select the best frame. Instead, the frame selector can only utilize frames received during the selection interval to perform the frame selection process. As new backhaul transport types (e.g. Ethernet and T1/E1-based IP backhaul or WiMax based IP backhaul) are introduced, the variations of the backhaul delay can be significantly larger than the delay of frame relay backhauls. As such, the differential delay among the new backhaul types and that between the new and the traditional frame relay backhaul can exceed the current limit for frame selection. Because of the expanded one-way and differential delays, calls and call legs are more likely to drop when the user carrying a mobile communication device in an active call moves across the boundary areas, as reverse frames from these “new backhaul” legs may arrive at the frame selector too late for frame selection, as well as to provide prompt feedback to the base stations.